1. Field of the Invention.
The present invention relates to grouwth of films of material, and, more particularly, to the growth of diamond-type films and related devices.
2. Description of the Related Art.
Diamond is a useful industrial material, providing hard surfaces for cutting tools, coatings for infrared optics, and thermally conductive electrical insulators for electronic devices. Synthetic diamonds have been produced under high-pressure and high-temperature conditions since 1955; and polycrystalline diamond films can be grown at moderate temperatures and pressures. See D. Vitkavage et al, Plasma Enhanced Chemical Vapor Deposition of Polycrystalline Diamond and Diamond-like Films, 6 J. Vac. Sci. Tech. A 1812 (1988). U.S. Pat. Nos. 3,030,187 and 3,030,188 disclose pyrolysis of hydrocarbon gases to deposit diamond despite the thermodynamic preference for graphite formation by including hydrogen gas which preferentially reacts with graphite and removes it. Similarly, diamondlike films, which are amorphous and contain a large fraction of carbon bonds in the sp.sup.2 configuration, can be formed by rf plasma deposition, low-energy ion beam deposition, dc glow discharge deposition, and sputtering. See J. Angus el al, Dense "Diamond-like" Hydrocarbons as Random Covalent Networks, 6 J. Vac. Sci. Tech. A 1778 (1988). However, diamondlike films have inferior hardness for use as cutting tool coatings and have inferior thermal conductivity for use with electronic devices.
Several known methods of growth of diamond films on non-diamond substrates have the problem of formation of nucleation sites. The most common substrate preparation procedure includes abrasion with diamond grit. It is believed that small embedded diamonds act as nucleation sites for the subsequent diamond film growth. But even with diamond-grit-abrasion substrate preparation, grown diamond "films" more closely resemble loose piles of individual diamonds; the nucleation density is apparently too low to readily from a continuous diamond film. This problem is especially severe for those growth conditions that produce the most perfect diamond, as gauged by SEM and Raman spectra.
Co-pending U.S. patent application Ser. No. 231,750 discloses a method for forming a continuous, good quality diamond film. FIGS. 2a-d schematically issustrate the steps of diamond film growth disclosed in the co-pending application. In FIG. 2a, silicon substrate 102 has been abraded with a fine diamond grit, leaving small embedded diamonds in surface 104 of silicon substrate 102, which act as nucleation sites 106. Substrate 102 is then inserted in to a deposition reactor and conditions sufficient for growth of a diamondlike material are introduced in the reactor, resulting in diamondlike layer 114, as shown in FIG. 2b. Diamondlike layer 114 is then subjected to an atomic hydrogen etch, which preferentially etches graphitically-bonded carbon, leaving the surface of the layer with a high density of nucleation sites 108, as shown in FIG. 2c. This is followed by introduction of growth conditions in the reactor sufficient to form diamond material, resulting in the growth of good quality, continuous diamond film 116, as shown in FIG. 2d.
However, analysis has shown that the electrical resistivity of even good quality, continuous diamond thin films is significantly less thah that of bulk diamond material. Although the resistivity of bulk Type I diamonds is in the 10.sup.12 Ohm cm range, the reported resistivity of thin film diamond is typically less than 10.sup.8 Ohm cm. The low electrical resistivity of diamond films has been attributed to space charge limited current; see Ashok, 50 Appl. Phys. Lett. 763 (1987) and Gildenblat, 53 Appl. Phys. Lett 586 (1988). Others have increased diamond resistivity by lower deposition temperature and attributed low resistivity to band bending at the grain boundaries; see Sokolina, 24 Inor. Mat. 1040 (1989). As diamond films gain increasing use in commercial applications, control of electrical resistivity characteristics will become increasingly necessary.